Effect of sodium montmorillonite clay on the kinetics of CH4 hydrate - Implication for energy recovery

Abstract The study of soil cracking is becoming increasingly important because of rising global temperatures and frequent natural disasters. And it poses a huge threat to the infrastructure. In the literature, soil cracking has been tackled by many researchers with various methods. The desiccation cracking process is influenced by various factors, such as soil thickness, temperature, and wet-dry cycles, which makes the process extremely complex. A large number of theoretical studies, indoor experiments and numerical simulations have been carried out. Many advanced techniques, such as X-ray CT and image processing, have also been used in this process. In this paper, the research work of soil cracking is briefly reviewed and summarized, including cracking mechanism, influencing factors of cracking, the comparison of different research methods. The review also lists current limitations and some promising directions for future research.

Mechanism, influencing factors and research methods for soil desiccation cracking: a review

The pre-peak microstructure evolution of the material is crucial in determining the strength and failure scheme of the material, but is not captured by the existing shear strength parameters models which focus on post-peak stage information. Novel concepts of pseudo-cohesion and pseudo-internal friction angle that account for both the pre-peak and post-peak stage information were proposed in this study, based on the experimentally observed response of the rock mechanics parameters to an external load (the maximum principal stress). These two parameters were analytically derived as functions of maximum principal strain, with coefficients to be determined (i.e., by applying the Mohr–Coulomb criterion with these parameters to experimental data). The dependence of pseudo-cohesion and pseudo-internal friction angle on the axial strain and confining pressure was investigated. The evolution of the strength parameters was found to relate to the propagation of micro-crack developed in the loading process, as revealed by PFC2D simulations reproducing experiments. The results show that (1) analytical model characterizing the pseudo-cohesion and internal friction angle as the quadratic function of axial strain accurately reflects the mechanical response characteristics of the specimen; (2) before the residual stage, the pseudo-cohesion first increases and then decreases with increasing strain, while the pseudo-internal friction angle increases continuously; (3) for the sandstone specimens studied, the pseudo-cohesion at the peak stress increases by a gradually decreasing rate with increasing confining pressure, while the pseudo-internal friction angle at the peak stress decreases by a decreasing rate with the increasing confining pressure; (4) CPM well characterized the mechanical properties of rock under multiple confining pressures, as the total stress–strain curves and failure modes of numerical specimens reproduced the experimental results. The evolution of rock pseudo-shear strength parameters is closely related to the external load and the generation, propagation, and penetration process of rock cracks. The rock pseudo-shear strength parameters are gradually mobilized with the increasing load, and the propagating of cracks owing to the decrease of mobilizable maximum cohesion and the increase of mobilizable maximum internal friction angle. 

/pdf/a-novel-obtaining-method-and-mesoscopic-mechanism-of-pseudo-3byrzxqw.pdf

A novel obtaining method and mesoscopic mechanism of pseudo-shear strength parameter evolution of sandstone

This study investigates the cyclic freeze-thaw (F-T) effect on crack patterns in saline clayey soil. Samples with different NaCl concentrations (0 M, 1 M, 2 M, and 3 M) are prepared and subjected to cyclic F-T processes. Each cycle involves freezing the samples at a constant temperature of −20 °C for 24 h and thawing at room temperature (21 °C) for 24 h. Additionally, a parallel experiment is conducted to observe the effects of desiccation at room temperature. The weight and surface crack patterns of the samples are continuously monitored. The image processing technique quantifies crack morphology and determines crack area, surface crack ratio, crack width, crack length, and fractal dimension. The results reveal that as the salt concentration increases, the F-T and desiccation samples exhibit decreased crack area and width. This decrease is attributed to the reduced saturation vapor pressure caused by higher salt concentrations, which slows down water evaporation and delays crack formation. The final crack pattern in the F-T samples is closely linked to the initial pattern of ice crystal formation. Lower salt concentrations result in the formation of larger ice crystals, leading to larger cracks during thawing. In the experiment, where the temperature was above the NaCl eutectic point, only ice crystals were formed, and the salt remained isolated as the salt concentration increased in the unfrozen solution. Conversely, during the freezing process, samples with higher salt concentrations with higher unfrozen solution content experience upward movement of the solution due to temperature gradients and water evaporation. This study provides new insights into understanding the soil water and salt movement and the phenomenon of surface clay soil cracking during the freezing-thawing process. 

Investigating cracking behavior of saline clayey soil under cyclic freezing-thawing effects

Salinity is commonly considered to be the main factor determining the crack extent of clayey soda saline-alkali soil in the Songnen Plain in China. However, there are great variations among different parameters in quantifying the extent of desiccation cracks in salt-affected soil, leading to substantial uncertainty in their abilities to characterize soil salinity. This study aimed to analyze the relationships between different types of crack parameters and the salinity of soil samples to establish optimal prediction models for the electrical conductivity (EC) values. To achieve these objectives, 104 soil samples with different salinity levels were obtained and pre-processed based on a unified standard for binary crack patterns. Thereafter, three common types of crack parameters including the crack intensity factor (CIF); box-counting dimensions under equal-partitions of 2, 3, 5, and 7; and four gray-level co-occurrence matrix (GLCM) texture features were extracted for correlation coefficients with the EC values of the soil samples based on a series of regression analyses. The results indicate that although different types of crack parameters all have a clear correlation with EC, CIF shows a poor relationship with EC values compared to those of the fractal dimensions and the GLCM texture features. The CIF, box-counting fractal dimensions based on 2 equal-partition, and contrast (CON) texture feature were then selected as the most ideal indices to develop exponential regression models for EC estimation. In addition, fitting results from verification work showed that CON was the best predictor of EC (R2 of 0.85), with an accuracy much higher than box-counting dimension based on 2 equal-partition (R2 of 0.68) and CIF (R2 of only 0.56). 

Comparative study on the abilities of different crack parameters to estimate the salinity of soda saline-alkali soil in Songnen Plain, China

On the whole, if the climate of the saline soil distribution area is arid or semi-arid, with small precipitation and large evaporation. Thus, soil desiccation cracks are a common phenomenon in saline soil. The cracks of soils cause serious problems in several fields (e.g., agriculture, geotechnical engineering, and engineering geology). The type and amount of salt in saline soil have important effects on its mechanical and hydraulic characteristics and cracking behavior. The purpose of this study is to quantitatively analyze the effects of salt content on the characteristics of crack development and water evaporation in clay. Laboratory experiments were carried out on six groups of samples with thin layers of saturated clay slurry with NaCl content of 0, 1%, 2%, 4%, 8%, and 16% by weight of dry soil under controlled temperature and humidity. During the drying process, the mass of the samples was recorded using an electronic balance, and photographs were taken with a digital camera to record the development of surface cracks. The geometric parameters of the crack images, such as the crack ratio and the total length of the crack network, were determined using digital image processing technology. Combined with the fractal theory, the development process of cracking was analyzed quantitatively. The results showed that water evaporation and crack development were both reduced with increases in NaCl content. The evaporation process of water could be divided into three stages: the steady, falling, and residual rate stages. With increases in NaCl content, the crack ratio, total length of the crack network, average crack width, and fractal dimension of the crack all decreased. When the NaCl content was equal to 16%, the surface of the soil was covered by a layer of salt due to salt crystallization, and no cracks appeared on the surface to the end of the evaporation process. The study results would contribute to hydrological analyses in arid and semi-arid lands with potential applications in water and land resources management.

Effects of salt content on desiccation cracks in the clay

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Effects of salt content on desiccation cracks in the clay